Increasing reliance on sophisticated electronic technologies has exacerbated a need for reliable, portable sources of energy. The current worldwide market for rechargeable batteries exceeds $4 billion. However, an estimated $6 billion market in the United States alone remains untapped due to the inability of available products to meet the needs of a large industrial, consumer, and military based market. And the need is growing, particularly for high power applications such as electric vehicles and load-leveling equipment and weight or volume constrained applications such as portable computers, cellular phones, etc. Creative alternatives to current battery technologies must be developed.
Among available technologies, lithium and lithium-ion batteries represent the most promising technologies, especially in terms of their attractive energy densities when compared to nickel cadmium and nickel-metal hydride cells. This promise can be attributed in large part to the extremely high specific capacity available in theory from lithium (3862 A h/kg) and the reasonable charge capacity available from lithium-intercalated carbon (e.g. for LiC6, 372 A h/kg) anodes.
However, realization of the full potential of that technology has been awaiting development of 1) electrolyte compositions able to withstand the high reactivities of those anodes, and 2) inexpensive stable cathodic materials also able to store high charge capacities. The present invention addresses resolution of the latter issue.
Batteries currently marketed utilize LiCoO2 cathodes, a material that has a theoretical specific capacity of 274 A h/kg and a practical specific capacity of 100 to 150 A h/kg. In realistic terms, then, cells using LiCoO2 can utilize only one-thirtieth of the potential energy per gram stored in lithium metal (or only one-third of the potential energy per gram stored in LiC6). While batteries based on such cathodes are suitable to meet short-term commercial needs, significant progress in this area must be based around developing cathodes with specific capacities similar to those of the best available anodes.
In terms of alternative cathode materials being currently studied, sulfur and sulfur containing compounds offer the best opportunities in terms of high theoretical specific capacities. Sulfur cathodes operate by storing energy as disulfide bridges, a very stable oxidized material, then releasing it by reducing to thiolate moieties. This process is illustrated in FIG. 1 for one material. While sulfur cathodes were the first employed for lithium laboratory cells, their use in practical batteries was abandoned primarily due to the insulating nature of those materials, which necessitates using high temperatures in order to facilitate mass transport within the cathodes.
However, advances in understanding and controlling electronic conductivity in composite materials has signaled a revival of that class of materials, as evidenced by ongoing exploratory or commercial efforts. Experimental lithium metal and lithium ion anode prototypes of systems recently developed match or exceed current room temperature secondary battery performances in terms of gravimetric energy density (energy delivered per mass of the cathode material).
In U.S. Pat. No. 5,789,108, Chu is pursuing technology based on cathodes composed of inorganic sulfur mixed with poly(ethylene oxide) (PEO), lithium salts and carbon powders, while Skotheim et al. in U.S. Pat. No. 5,529,860, are developing cathodes based on inorganic sulfur and poly(acetylene) derivatives. Those formulations represent attempts at achieving electronic and ionic access to the sulfur in those cathode via dispersion in an ionically conducting medium (PEO) which contains an electronic conductor (carbon)(Chu) or the use of a conducting polymer to effect both goals (Skotheim et al.).
Although such materials show excellent specific capacities during initial cycles, neither Chu nor Skotheim et al. have commercialized a sulfur or organosulfur based secondary battery due to high capacity loss, or “fade: observed for their cells over multiple charge and discharge cycles. Consideration of the possible origins of that behavior are obscured by a general lack of knowledge regarding the complicated electrochemistry of inorganic sulfur, compounded by the varying solubilities and chemical activities of the different forms of sulfur (e.g. polysulfide, oligosulfide, dimer, monomer (S2−)), possible reaction between those species and the anode, and the likely growth of heterogeneous zones of non-conductive material that separate from the initial cathode mixture.
However, it is clear from recent work that the capacity fade observed is due in large part to diffusion of monomeric sulfur species out of the cathode. While interesting redox chemistry is typically evident in the first few cycles, it typically fades after several cycles to leave less desirable redox activity—viz, the organosulfur or sulfur or selenide component exits the cathode via dissolution into the electrolyte film, resulting in capacity loss during cycling.
In WO 99/03162, Skotheim et al. describe a polymer film that protects the lithium anode in a secondary battery. The film interferes to some degree with the reactions and thus reduces the overall efficiency of the battery. The film is needed when sulfur species are mobile and may migrate to the anode, Skotheim et al. do not suggest decreasing or eliminating the mobility of reactive sulfur species so that interference at the anode is decreased and thereby the efficiency of the battery is increased.